This invention is concerned with improvements in excavation buckets for earthworking apparatus.
This invention is concerned particularly although not exclusively with a light-weight high capacity bucket for a dragline excavation.
Dragline excavation apparatus represents a very large capital expenditure of the order of hundreds of millions of dollars and operating overheads can cost in excess of US $3000 per hour. Accordingly it is necessary to maximise the efficiency of such draglines as they operate 24 hours a day, 7 days a week. Any reduction in operational efficiency can represent substantial annual productivity losses when it is considered that a typical excavation payload is about 80 tonnes of earth.
Generally speaking, most dragline excavators are a compromise between competing factors. While a longer boom length may improve operational efficiency by providing an increased dump radius, the load capacity of the boom is reduced proportionately. Similarly, bucket/rigging combinations of reduced mass may permit increased payloads but this is usually at the expense of bucket control and increased maintenance and downtime costs.
A number of characteristics are employed to determine operational efficiencies of dragline buckets. These include:
DRAG ENERGY: A measure of the energy required to fill a bucket of a given capacity. Factors affecting drag energy include the extent of frictional engagement between internal and external bucket surfaces and earth masses within and without the bucket respectively, tooth/cutting edge configuration and dead weight of the bucket/rigging combination,
TOTAL SUM LOAD (TSL): The sum of the masses of the bucket, the rigging and the payload.
Over the years there have been many modifications to excavator bucket designs and rigging configurations to achieve at the same TSL a greater payload as a proportion of TSL and/or to reduce the cycle time of the bucket.
U.S. Pat. No. 2,334,460 describes an archless bucket with a substantial lip casting which extends up to the top of the side walls to support pivotal drag chain couplings thereon. The side walls and rear wall, all of substantially the same height, are generally planar and diverge outwardly at their upper edges. This configuration is considered to have reduced volumetric capacity and is not suited to heavy service life due to its inefficient lip design and flimsy construction necessitating internally mounted hoist rope mountings which are subject to increased wear.
U.S. Pat. No. 3,247,606 describes a high backed bucket which may be constructed with or without a front arch. This bucket configuration is considered to be inefficient in terms of the drag energy required to fill the bucket and material losses out of the front of the bucket during the initial hoist operation. An exaggerated rear tilt carry angle is required to reduce spillage but this may lead to instability in the bucket with difficult or premature dumping.
U.S. Pat. No. 5,400,530 describes an archless bucket with a low tapered rear wall for rearward dumping. The hoist ropes are mounted on slidable trunnions located within the bucket and secured to a very substantial lip casting with integral cheek plates to which the drag ropes devises and the slidable hoist rope trunnions are secured. This bucket is not considered to be sufficiently durable for sustained heavy duty operation nor is it considered to be sufficiently stable for optimised operations.
Another high backed archless bucket is descried in U.S. Pat. No. 3,247,607. The combination of bucket and the rigging system employed to achieve the desired carry angle are considered to add substantially to the xe2x80x9cdeadxe2x80x9d portion of an excavator payload.
U.S. Pat. Nos. 4,791,738, 5,575,092, 3,597,865, 5,343,641, 5,307,571 and 5,140,761 are illustrative of contemporary bucket/rigging systems which, while durable, are very heavy and limit operational efficiencies of a dragline excavator.
U.S. Pat. No. 4,944,102 addresses certain of the problems with prior art excavator bucket designs and claims to offer improvements thereover. This patent particularly addresses the problem with prior art buckets having a back wall approximately the same height as the side walls wherein excess time and energy is spent endeavouring to fill the bucket completely. The problem of excessive bucket and rigging mass is said to be overcome by reducing the height and amount of material in the back wall and locating the hoist rope trunnions inwardly of the bucket side walls thereby eliminating the need for a spreader bar. On balance, It is considered that this bucket/rigging assembly would offer only marginal, if any, improvement in operational efficiency compared with contemporary prior art systems.
A low draft, high yield bucket system is described and illustrated in U.S. Pat. No. 5,832,638 and U.S. Design Pat. Des No. 392,983. These documents show an upwardly sloping lip leading to a planar floor and outwardly divergent side walls which taper rearwardly and which have an upright portion extending longitudinally thereof. A rear wall has a lower outwardly divergent portion and an upper inwardly convergent portion. Lip end cutters extend forwardly and upwardly from the front cutting lip and diverge outwardly at the same angle as the outwardly divergent portion of the side walls. This bucket is claimed to be about half the weight of a conventional bucket as less wear plates are required to address the problem that the sides, backs and bottoms of prior art buckets did not fit the cleave planes in the earth mass behind the front lip. With prior art buckets it is Indicated that interior wear plates are required as the earth is extruded with great force into a conventional bucket having a generally D shaped cross section with straight sides.
In a conventional bucket it is claimed that there is little or no increase in cross section within the bucket to allow lateral swell of excavated material which is squeezed and forced upward at a great loss of drag energy.
In use, The bucket of U.S. Pat. No. 5,832,638 is said to provide an optimised loading distance of approximately 3 bucket lengths compared with 1.5 to 2 lengths of a conventional bucket. This bucket is also claimed to tilt, under the influence of a tilt back system to a precise preset tilt back angle relative to the stops in the dump line. In so doing the bucket spills excess earth over the back and sides and leaves the bucket with a relatively predictable weight to match the operating weight of the dragline. This spillage is considered to largely negate any of the claimed benefits of this bucket and rigging assembly.
It is an aim of the present invention to provide an improved excavator bucket which overcomes or alleviates at least some of the disadvantages associated with prior art excavator buckets.
According to one aspect of the invention there is provided a dragline bucket comprising:
a floor, opposed side walls and a rear wall;
a lip extending transversely of a front portion of said floor, said lip being adapted to receivably locate a plurality of spaced cutting teeth;
opposed cheek plates adjacent respective front portions of said side walls, said cheek plates each being adapted for attachment of rigging members; and,
hoist rope attachment members located intermediate front and rear portions of said bucket, said bucket characterised in that said rear wall has a height greater than adjacent side wall portions and respective junctions between said floor and said side walls each are formed with an arcuate inner surface.
Suitably said rear wall tapers upwardly from said floor.
If required said rear wall may taper upwardly from said floor in an arcuate manner.
The rear wall may include a flange member extending about an upper edge thereof.
Suitably the flange member extends from respective upper edges of said side walls about said rear wall.
Preferably said flange member extends inwardly of at least said rear wall.
If required, front portions of respective side walls may be of a height greater than respective side wall portions adjacent said rear wall.
The lip may include spaced tooth adaptors formed integrally therewith.
Alternatively the spaced tooth adaptors may be removably secured to said lip.
Suitably said tooth adaptors comprise convergently tapered upper and lower faces adapted to engage in respective tapered sockets of teeth removably secured thereto.
If required said teeth may be secured to said adaptors by screw threaded members threadably locatable in said tooth.
Suitably respective junctions between said cheek plates and adjacent floor and/or front lip portions are formed with an arcuate inner surface.
The cheek plates may be formed separately from or integral with said front lip.
Suitably, leading edge portions of said cheek plates are inclined forwardly from respective lower regions thereof.
Preferably said cheek plates are adapted to receivably locate removable wear plates and/or cutting edges.
If required, the arcuate inner surfaces formed at the respective junctions of side walls and the floor of said bucket are formed as shaped junction members adapted for attachment to respective side walls and the floor.
The junction members may extend to form respective junctions of said side walls and said rear wall.
Suitably said junction members each comprise one or more castings.
If required said hoist rope attachment members may be attached to respective junction members.
Preferably said hoist rope attachment members are formed integrally with respective junction members.
Suitably said side walls and said floor in plan view taper rearwardly towards said rear wall.